Synchronous Motor Question ?

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SG-1

SG-1

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I am looking at a motor control circuit for a 4160V 4000 HP brushless synchronous motor. The exciter circuit is manual in that the voltage is set with a big knob attached to a variable auto-transformer. The operator has a AC ammeter 0-600A, a DC Voltmeter, a DC ammeter with high/low alarm contacts, & a Power Factor meter to monitor.

The circuit is using a MOC from the feeder breaker to apply the field current, so when the full line voltage is applied the field current is also applied, if the knob is not set back to zero when the motor is stopped. No speed switch or timer is used.

The 55 has an adjustable 0-10 second delay. A SEL 710 is being used for the main motor protection.

Is it very common for an operator to manually adjust the field current from 0 to 10A when starting a sync motor ? I am used to seeing this done with a speed switch or timer.

What would happen if the the excitation is applied at the same moment as the primary voltage ?? Million dollar disaster ?
 
T

T.M.Haja Sahib

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If the syn. motor has no damper winding on its rotor, the motor would not start on application of the excitation at the same moment (or later) as the primary voltage. If the syn. motor is provided with a damper winding on the rotor, it is started as an induction and then pulled into synchronism, when excitation is applied. I do not think any harm may arise when both excitation and primary voltage are applied at the same time except that its starting current is higher to be taken care of adequate size of fuse/breaker.
 
T

T.M.Haja Sahib

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However, I want to add that starting a syn motor by applying both excitation and primary voltage at the same time is not a good practice, anyway because it creates a temporary power surge. Your apprehension is justified.
 
Jraef

Jraef

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Agreed. MOST (although not all) synch. motors have what is called an "Amotissuer" winding that is used to accelerate the motor. Certainly anything new enough to have brushless excitation is going to be like this. This is essentially an induction motor inside of the synch. motor, or in other words it's almost as if you have two machines connected to the dame shaft. So think of it this way, if you have an induction motor spinning a synchrounous machine, and the windings of the synchronous machine are excited, what do you have? An Alternator! But in this case, the alternator and the induction motor are connected to the line at the same time. Bad news...
 
SG-1

SG-1

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Location
Ware Shoals, South Carolina
Thanks to all who replied. I have been assured that the actual application section is rotating on the motor & will control when the DC is applied. The MOC is turning on the the DC supply a few seconds ahead of when it is needed.

Continue with the super sync motor Phil.
 
T

T.M.Haja Sahib

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SG-1 said:
The MOC is turning on the the DC supply a few seconds ahead of when it is needed.
Click to expand...

The good practice is to switch on the DC supply after the motor speed is within 1% to 5% of the synchronous speed to prevent temporary current surge in the lines connecting to the motor as already mentioned. Most syn. motors have automatic control starting to insure that the DC excitation is connected to the rotor at the most opportune time.
 
T

T.M.Haja Sahib

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Phil Corso said:
TM...

No!

Phil
Click to expand...

Yes. You are correct. You are describing a different syn motor developed by GE. The stator as well as rotor can rotate in that motor. The stator first rotates on starting at synchronous speed. Its speed is gradually reduced by means of a brake and a corresponding speed is picked by the rotor connected to heavy load. Braking of the stator continues till it is completely stopped and the rotor revolves at the synchronous speed. The main advantage of motor? High starting torque as you already described. Main disadvantage? Costliest of its kind.
Super synchronous motor is a misnomer because it does not operate at super synchronous speed; Super torque syn motor may be the correct name.
 
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Phil Corso

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Boca Raton, Fl, USA
TM... Excellent!

The only difference between it and the "ordinayr" synchronous motor was that the stator was provided with trunnion bearings allowing it to rotate as well as the rotor. Hence during start, rotor and driven-machine were stationary and stator accelerated to syncronous speed.

After appplication of rotor excitation, a band-brake encircling stator was tightened. Breakaway-torque was sufficient to rotate rotor and driven-load. There were 5 slip rings; 3 for stator and 2 for rotor

Developed in the '40s, I believe the term "super-synchronous" was patented by GE. The one I was invoved with when I worked for US Gypsum, was about 600 Hp, 440V. The driven-machine was a ball-mill!

Yes it was expensive, but the chief advantage was elimination of a very large electrrically-operated coupling! The result was a significant reduction in the lenght of the machinery-train ('foot-print', today!)

TM, where did you find info you cited?

Regards, Phil
 
SG-1

SG-1

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Location
Ware Shoals, South Carolina
T.M.Haja Sahib said:
The good practice is to switch on the DC supply after the motor speed is within 1% to 5% of the synchronous speed to prevent temporary current surge in the lines connecting to the motor as already mentioned. Most syn. motors have automatic control starting to insure that the DC excitation is connected to the rotor at the most opportune time.
Click to expand...

By "turn on" I mean the DC supply is made available & stable for the application at the appropriate time. We are only supplying the variable DC power supply. In the past we have provided a speed switch also.
 
T

T.M.Haja Sahib

Guest
Phil Corso said:
TM...

Dr. Kosow was my mentor early in my career! His book was the text for an Electrical-Technology course I taught in a NYC Community College!

Do you have the entire article?

Phil
Click to expand...
No. I do not have the book. Since you also taught the course, I request you to explain from the first principles the mechanism behind the rotor revolving in the opposite direction to that of stator during starting of the super synchronous motor for keeping the combined speed of stator and rotor constant at the synchronous speed. Thanks.
 
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